The invention relates to an oscillator for controlling a mixer stage in a tuner, which oscillator comprises a parallel resonant circuit including a variable capacitance which is variable in dependence upon a fine-tuning control signal and is constituted by capacitance occurring in the collector circuit of a transistor, and is variable in dependence upon a fine-tuning control signal applied to an electrode pair of the transistor. A tunable oscillator of this type is known from U.S. Pat. No. 4,034,298 in which the oscillator transistor itself constitutes the variable capacitance in that the capacitance of its base-collector diode is varied by means of a fine-tuning control signal which is applied to the collector of the transistor. In accordance with the working point required for oscillator operation of the transistor, its base-collector diode is always cut off so that only relatively small capacitance variations can be achieved.
It is an object of the invention to improve the turnablility of an oscillator of the type described in the opening paragraph in such a way that a large and stable variation range of the capacitance requried for fine tuning is achieved, also at relatively low values of the supply voltage for the circuit arrangement.